The acetabular labrum seal: a poroelastic finite element model

The management of labral tears and femoroacetabular impingement of the hip in the young, active patient

PURPOSE

The purpose of this systematic review was to determine (1) the quality of the literature assessing outcomes after surgical treatment of labral tears and femoroacetabular impingement (FAI), (2) patient satisfaction after open or arthroscopic intervention, and (3) differences in outcome with open or arthroscopic approaches.

METHODS

Computerized literature databases were searched to identify relevant articles from January 1980 to May 2008. Studies were eligible for inclusion if they had a level I, II, III, or IV study design and if the patient population had a labral tear and/or FAI as the major diagnosis. Patients with severe pre-existing osteoarthritis or acetabular dysplasia were excluded.

RESULTS

Of the 19 articles with reported outcomes after surgery, none used a prospective study design and only 1 met the criteria for level III basis of evidence. Open surgical dislocation with labral debridement and osteoplasty is successful, with a good correlation between patient satisfaction and favorable outcome scores. The studies reviewed support that 65% to 85% of patients will be satisfied with their outcome at a mean of 40 months after surgery. A common finding in all series, however, was an increased incidence of failure among patients with substantial pre-existing osteoarthritis. Arthroscopic treatment of labral tears is also effective, with 67% to 100% of patients being satisfied with their outcomes.

CONCLUSIONS

The quality of literature reporting outcomes of surgical intervention for labral tears and FAI is limited. Although open surgical dislocation with osteoplasty is the historical gold standard, the scientific data do not show that open techniques have outcomes superior to arthroscopic techniques.

LEVEL OF EVIDENCE

Level IV, systematic review.

Arthroscopic management of femoroacetabular impingement: osteoplasty technique and literature review

Morphological and spatial abnormalities of the proximal femur and acetabulum have been recently recognized as causes of femoroacetabular impingement. During joint motion in hips with femoroacetabular impingement, abnormal bony contact occurs, and soft tissue structures (chondral and labral) often fail. Femoroacetabular impingement has been reported to be a contributor to early-onset joint degeneration. Ganz et al have described good midterm success with an open surgical dislocation approach to reconstruct normal joint clearance. The purpose of this report is to discuss relevant literature and describe an arthroscopic approach to treat femoroacetabular impingement. This approach has particular relevance in high-demand patients, particularly in athletes seeking to return to high-level sport.

A finite element exploration of cartilage stress near an articular incongruity during unstable motion

Both instability and residual articular incongruity are implicated in the development of post-traumatic osteoarthritis (OA) following intra-articular fracture, but currently no information exists regarding cartilage stresses for unstable residual incongruities. In this study, a transversely isotropic poroelastic cartilage finite element model was implemented and validated within physiologically relevant loading ranges. This material model was then used to simulate the loading of cartilage during stable and unstable motion accompanying a step-off incongruity residual from intra-articular fracture, using load data from previous cadaver tests of ankle instability. Peak solid-phase stresses and fluid pressure were found to increase markedly in the presence of instability. Solid-phase transients of normal stress increased from 2.00 to 13.8 MPa/s for stable compared to unstable motion, and tangential stress transients increased from 17.1 to 118.1 MPa/s. Corresponding fluid pressure transients increased from 15.1 to 117.9 MPa/s for unstable motion. In the most rapidly loaded sections of cartilage, the fluid was found to carry nearly all of the normal load, with the pressurization of the fluid resulting in high solid matrix tangential stresses.

Arthroscopic repair of the acetabular labrum: a histologic assessment of healing in an ovine model

PURPOSE

Recently, arthroscopic repair has been advocated for the treatment of labral tears. The purpose of this study was to document the histologic healing process in an arthroscopically repaired acetabular-labral detachment in an ovine model.

METHODS

A total of 10 skeletally mature female sheep underwent unilateral arthroscopic labral repair. A 1.5-cm-long incision was made at the junction of the labrum and acetabulum. The labral detachment was then repaired arthroscopically with a single suture anchor. Postoperatively, the animals were confined to small indoor pens, and no form of immobilization was used. After 12 weeks, the animals were killed and the labral repair evaluated via routine histology. Normal labra from 3 nonoperative animals were also processed for histology.

RESULTS

The acetabular labrum of the sheep is an avascular, fibrocartilaginous structure similar in anatomic design to that of human beings. All arthroscopically repaired labra appeared stable and grossly healed at 12 weeks. Histologically, labral healing appeared to occur via fibrovascular scar tissue originating from the joint capsule or the exposed bony attachment of the labrum (or both). However, the labral healing was incomplete in all specimens, with a shallow, superficial cleft remaining at the junction of the labrum and the articular surface of the acetabulum.

CONCLUSIONS

Arthroscopically repaired acetabular-labral lesions in sheep are capable of healing via fibrovascular repair tissue or direct reattachment via new bone formation (or both).

CLINICAL RELEVANCE

The ability of labral detachments to heal in this animal model provides the rationale for additional investigations into the clinical efficacy of arthroscopic labral repair in human beings.

A new method for acetabular rim trimming and labral repair

In the past, arthroscopic technique for the treatment of a torn acetabular labrum was limited to simple debridement of the torn tissue. However, due to recent advancements in the understanding of the function of the labrum, it has been suggested that labral repair can be beneficial. The purpose of this article is to report a new technique for arthroscopic rim trimming of pincer-type femoroacetabular impingement and labral repair through the lateral arthroscopic portal.

Labral tears, extra-articular injuries, and hip arthroscopy in the athlete

Labral tears in athletes can lead to disabling hip pain and affect their athletic performance. Other intra-articular lesions, including chondral injuries, capsular abnormalities, and ligamentum teres tears, commonly coexist with acetabular labral tears. Isolated athletic injury or repetitive traumatic activity can lead to labral tears; however, underlying structural (femoroacetabular impingement) and developmental abnormalities predisposing athletes to labral pathology must be addressed. Recent studies have demonstrated lesions associated with acetabular labral tears, and that labral tears rarely occur as isolated injuries. Return to sport is favorable in athletes who have labral tears if they are properly treated with arthroscopic intervention.

[Femoroacetabular impingement: trigger for the development of coxarthrosis]

Femoroacetabular impingement (FAI) is frequent; the estimated prevalence ranges between 10 and 15%. Our 10-years experience strongly suggests that FAI leads to osteoarthritis. Isolated acetabular or femoral abnormalities are rare, even though in women acetabular and in men femoral abnormalities predominate. Normal radiographs do not exclude the presence of FAI. Symptoms are related to the degree of deformity and occur earlier in the presence of activities requiring high levels of motion. The majority of patients with FAI are under the age of 40 years. In contrast to impingement in total hip replacement, the natural hip is under much higher constraint, not allowing to escape from impingement-induced shear forces by subluxation or complete dislocation. FAI-induced shear forces due to an aspherical femoral head/neck (cam type) are therefore high, causing outside-in damage with cleavage lesions of the acetabular cartilage by forced flexion and internal rotation. The cartilage of the femoral head remains initially intact, which cannot be explained by the classic concept of osteoarthritis. After the femoral head has migrated into the acetabular cartilage defect, vertical forces contribute to the further course of osteoarthritis. Tears between the labrum and cartilage, as seen by MRI, are not avulsions of the labrum from the cartilage but rather outside-in avulsions of the cartilage from the labrum. In acetabular overcoverage (pincer type) the labrum is the first structure to fail and acetabular cartilage damage develops thereafter. The treatment of FAI in patients under the age of 40 years is aimed at joint preservation. The clinical result is worse in the presence of significant cartilage damage. Therefore, early appreciation of FAI and timely therapeutic intervention as well as professional and athletic adjustment are important if osteoarthritis is to be prevented.

Repair of the adult acetabular labrum

Although hip arthroscopy is a new technique, indications for its use are rapidly expanding. With better utilization and understanding of both radiologic assessment of hip pathology and clinical examination, specific intra-articular derangement is being identified. Longitudinal, peripheral, and intrasubstance anterior hip labral tears can be repaired using anterior paratrochanteric and anterior portals with standard hip arthroscopy equipment. Repair of the torn labrum may help re-establish the anatomic function of the labrum, thereby preventing biomechanical compromise through surgical debridement, which may lead to degenerative changes associated with osteoarthritis. Although long-term results are still unknown, results of short-term follow-up are positive.

Diagnosis and Management of Traumatic and Atraumatic Hip Instability in the Athletic Patient

Although relatively uncommon compared with the shoulder, hip instability can be a source of significant disability and is a commonly unrecognized injury. Hip instability can be traumatic or atraumatic in origin. Our understanding and treatment plan for hip instability due to traumatic events is well established. However, our understanding and treatment modalities for hip instability due to atraumatic events or repetitive motion in high level athletes are not as well defined. In this article, we will review the spectrum of traumatic and atraumatic hip instability and discuss the relevant anatomy, history, and physical examination findings, imaging studies, and treatment options with a focus on hip arthroscopy, and review of the literature.

Neuromuscular Hip Biomechanics and Pathology in the Athlete

Although hip arthroscopic techniques have been developed and evolved over the last 5 to 10 years to help active athletes, the mechanisms of athletic hip injuries across various sports are not well understood. The purpose of this article is to review the literature related to the osseous and ligamentous support as well as the neuromuscular control strategies associated with hip joint mechanics. The neuromuscular contributions to hip stability and mobility with respect to gait will be provided because this data represents the largest body of knowledge regarding hip function. Further, this article will present and describe probable mechanisms of injury in sporting activities most often associated with hip injury in the young athlete.

MR imaging of femoroacetabular impingement

FAI is a common cause of OA of the hip. It can be caused by decreased offset of the femoral head and neck, resulting in abutment of the femoral neck against the acetabular rim (cam impingement) and abutment of this area into the well-constrained socket (pincer impingement)within normal range of motion [15]. This repetitive mechanical trauma to the hip joint causes mechanical wear of the labrum and articular cartilage; left untreated, it causes pain, labral tears, and chondral injuries leading to progressive OA of the hip. The identification of FAI as a cause of OA allows appropriate therapy early and thus delays or prevents end-stage arthritis.MR imaging and MR arthrography are accurate noninvasive imaging modalities able to demonstrate acetabular labral disease and adjacent cartilage damage as well as the subchondral cysts and synovial herniation pits associated with impingement. In addition, MR imaging is able to detect underlying subtle anatomic variations of the femoral head-neck junction and acetabulum associated with FAI.

Arthroscopic labral repair in the hip: surgical technique and review of the literature

Hip pain can be caused by multiple pathologies. Injuries to the acetabular labrum are the most common pathologic findings identified at the time of hip arthroscopy. Five causes of labral tears have been identified; these include trauma, femoroacetabular impingement, capsular laxity, dysplasia, and degeneration. Studies have shown the function and the importance of the labrum. To restore function of the labrum, new surgical techniques, such as suture anchor repair, have been described. The goal of arthroscopic treatment of a torn labrum is to relieve pain by eliminating the unstable flap tear that causes hip discomfort. The goals of these treatments are to maintain the function of the hip joint and decrease the development of premature arthrosis.

Imaging of the Acetabular Labrum

The acetabular labrum is a critical structure within the hip joint. It is a significant cause of pain, and it plays an integral role in the development of osteoarthritis. This article reviews the normal anatomy, imaging techniques, and normal MR appearance of the labrum. The clinical presentation of labral tears and their association with developmental dysplasia and femoroacetabular impingement are discussed. The controversy regarding the presence or absence of a sublabral sulcus also is addressed.

Hip arthroscopy update

The management of hip injuries in the athlete has evolved significantly in the past few years with the advancement of arthroscopic techniques. The application of minimally invasive surgical techniques has facilitated relatively rapid returns to sporting activity in recreational and elite athletes alike. Recent advancements in both hip arthroscopy and magnetic resonance imaging have elucidated several sources of intraarticular pathology that result in chronic and disabling hip symptoms. Many of these conditions were previously unrecognized and thus, left untreated. Current indications for hip arthroscopy include management of labral tears, osteoplasty for femoroacetabular impingement, thermal capsulorrhaphy and capsular plication for subtle rotational instability and capsular laxity, lateral impact injury and chondral lesions, osteochondritis dissecans, ligamentum teres injuries, internal and external snapping hip, removal of loose bodies, synovial biopsy, subtotal synovectomy, synovial chondromatosis, infection, and certain cases of mild to moderate osteoarthritis with associated mechanical symptoms. In addition, patients with long-standing, unresolved hip joint pain and positive physical findings may benefit from arthroscopic evaluation. Patients with reproducible symptoms and physical findings that reveal limited functioning, and who have failed an adequate trial of conservative treatment will have the greatest likelihood of success after surgical intervention. Strict attention to thorough diagnostic examination, detailed imaging, and adherence to safe and reproducible surgical techniques, as described in this review, are essential for the success of this procedure.

Vascularity of the hip labrum: a cadaveric investigation

PURPOSE

The purpose of this study was to define the vascularity of the hip labrum and to identify regional differences in vascular penetration that may have implications for the healing potential of this structure.

TYPE OF STUDY

Injection study of human cadavers to investigate the vascularity of the hip labrum.

METHODS

Twelve hips from 6 human frozen cadavers devoid of severe articular pathology were used. High-resolution surface-coil magnetic resonance imaging (MRI) was performed in both sagittal and coronal planes to better define anatomic planes in 2 dimensions and to correlate Spalteholz sections with the surrounding joint and labral anatomy. Each pelvis was injected with intra-arterial India Ink and frozen; 3-mm sagittal or coronal sections were then cut and processed using a modified Spalteholz technique, yielding anatomic zones of the labrum. Six specimens were cut in the sagittal plane and 6 were cut in the coronal plane. Specimens were examined at x10 magnification with transillumination. Anatomic zones with regional variations in vascularity were defined.

RESULTS

The anterior and superior aspects of the labrum showed degenerative changes on MRI and under direct manual transillumination in 75% of specimens. Overall, there was a relatively poor vascular supply to the labrum; however, there were regional differences between anatomic zones. Zone I (capsular contribution) had significantly more vascularity than zone II (articular side) (P < .01). Zone IA (the portion of the zone not attached to bone) showed the most consistent source of vessels across all specimens (smallest variation between specimens); however, zone IB (the portion of the zone attached to bony acetabulum) had the greatest overall mean vascularity score. These differences were not statistically significant. Furthermore, vascularity patterns were not significantly different in the anterior, superior, posterior, and inferior labral regions, nor were they different in torn versus intact specimens.

CONCLUSIONS

The cadaveric specimens evaluated in this study had a relatively avascular hip labrum. However, the increased vascularity seen in zone I (capsular side) may have implications for treatment, similar to that described in the meniscus of the knee.

CLINICAL RELEVANCE

A better understanding of the vascularity of the hip labrum will guide treatment of labral pathology and may have implications for the healing potential of this structure. Labral tears occurring in the vascular zone may be amenable to arthroscopic repair rather than debridement.

Histopathologic features of the acetabular labrum in femoroacetabular impingement

This paper summarizes clinical and histopathologic findings derived from 25 patients who had surgery for symptomatic femoroacetabular impingement. We explored if observed pathologic features were consistent with hypothesized mechanisms of injury, if severity of osteoarthritis and labral degeneration were associated, and if labral refixation would present an alternative. Clinically, all patients presented with limited range of motion at the hip attributable to pain and a positive impingement test. Magnetic resonance arthrography and surgical observations showed degenerated or ruptured labra or both in the anterior and/or superior regions of the acetabular rim (24 of 25 specimens) which correlated with pain provocation, limited range of motion, and anatomic deformities. Histologically, labra were mostly hyperplastic with disorganized cystic matrices. No inflammation was observed. Spatial distribution of degenerated labral matrices was not different for the two femoroacetabular impingement mechanisms. Labral degeneration and severity of osteoarthritis observed on radiographs did not correlate. In patients having only joint debridement, the labral matrix at the tip, near its vascular supply, was normal. Femoroacetabular impingement is a gentle chronic irritation of the labra located at the site of rupture that elicits a degenerative reaction. In early stages of the disease, the labral tip is not involved, providing the possibility of labral refixation after resection of the degenerated portion.

Effects of removal of the acetabular labrum in a sheep hip model

OBJECTIVE

The aim of this study was to learn whether partial resection of the acetabular labrum would lead to degenerative arthritis in an ovine model.

METHODS

A 2 cm segment of labrum was removed from one hip in 18 mature Swiss Alpine sheep and a sham procedure was performed on the opposite side. Animals were permitted ad lib activity until sacrifice at 6, 12 or 24 weeks. The hip joint was removed en bloc, and loaded with a force of one body weight, using a custom device designed to recreate a physiologic joint reaction force. While under load, the joint was plunge frozen, and then fixed by freeze substitution using aldehydes in methanol/acetone solvents. The entire joint was embedded in methacrylate and sectioned in a standardized frontal plane following the reaction force and including the femoral neck and the acetabular fossa. The sections were evaluated for concentricity and evidence of arthrosis. Six hip joints of three sheep with no surgical procedure were loaded with high or low loads and served as non-surgical controls.

RESULTS

Degenerative changes were present in all surgical hips, but the changes were symmetrical and mild. In 16 of 18 hips, the labrum regenerated to the extent that dense fibrous scar extending from the surgically denuded origin filled the defect.

CONCLUSION

Resection of the labrum does not cause rapid degeneration or altered stability of the sheep hip.

Hip arthroscopy: current indications, treatment options, and management issues

The management of hip injuries in athletes has evolved significantly in the past few years with the advancement of arthroscopic techniques. The application of minimally invasive surgical techniques has facilitated relatively rapid returns to sporting activity in both recreational and elite athletes. Recent advancements in both hip arthroscopy and magnetic resonance imaging have elucidated several sources of intraarticular abnormalities that result in chronic and disabling hip symptoms. Many of these conditions were previously unrecognized and, thus, left untreated. Current indications for hip arthroscopy include the presence of symptomatic acetabular labral tears, hip capsule laxity and instability, chondral lesions, osteochondritis dissecans, ligamentum teres injuries, snapping hip syndrome, iliopsoas bursitis, and loose bodies (for example, synovial chondromatosis). Less common indications include management of osteonecrosis of the femoral head, bony impingment, synovial abnormalities, crystalline hip arthropathy (gout and pseudogout), infection, and posttraumatic intraarticular debris. In rare cases, hip arthroscopy can be used to temporize the symptoms of mild-to-moderate hip osteoarthritis with associated mechanical symptoms. This article discusses the current clinical and radiographic methods to detect early hip joint disease and the current indications and surgical techniques of hip arthroscopy.

Acetabular labrum and its tears

Groin pain is a major cause of morbidity in athletes. Only in the last decade have acetabular labrum tears been recognised as a possible diagnosis. Awareness of this condition is important for appropriate management. The basic science and pathological and clinical features of acetabular labrum tears are reviewed, and diagnostic and treatment options are presented.

MR imaging of sports-related hip disorders

Hip arthroscopy is being used increasingly for the diagnosis and treatment of hip disorders. MR imaging performed with appropriate technical considerations may aid not only in preoperative planning but in the appropriate selection of patients, which tends to lead to better postoperative results. Although the painful hip is imaged most commonly by radiography, MR imaging is considered the next imaging test of choice for evaluation of most common hip abnormalities in athletes, including labral injuries, ligament injuries, osteochondral injuries, fractures, bursitis, and musculotendinous injuries. MR arthrography can be a particularly useful technique for dedicated assessment of hip joint internal derangements.

An in vitro investigation of the acetabular labral seal in hip joint mechanics

Labrum pathology may contribute to early joint degeneration through the alteration of load transfer between, and the stresses within, the cartilage layers of the hip. We hypothesize that the labrum seals the hip joint, creating a hydrostatic fluid pressure in the intra-articular space, and limiting the rate of cartilage layer consolidation. The overall cartilage creep consolidation of six human hip joints was measured during the application of a constant load of 0.75 times bodyweight, or a cyclic sinusoidal load of 0.75+/-0.25 times bodyweight, before and after total labrum resection. The fluid pressure within the acetabular was measured. Following labrum resection, the initial consolidation rate was 22% greater (p=0.02) and the final consolidation displacement was 21% greater (p=0.02). There was no significant difference in the final consolidation rate. Loading type (constant vs. cyclic) had no significant effect on the measured consolidation behaviour. Fluid pressurisation was observed in three of the six hips. The average pressures measured were: for constant loading, 541+/-61kPa in the intact joint and 216+/-165kPa following labrum resection, for cyclic loading, 550+/-56kPa in the intact joint and 195+/-145kPa following labrum resection. The trends observed in this experiment support the predictions of previous finite element analyses. Hydrostatic fluid pressurisation within the intra-articular space is greater with the labrum than without, which may enhance joint lubrication. Cartilage consolidation is quicker without the labrum than with, as the labrum adds an extra resistance to the flow path for interstitial fluid expression. However, both sealing mechanisms are dependent on the fit of the labrum against the femoral head.

The influence of the acetabular labrum seal, intact articular superficial zone and synovial fluid thixotropy on squeeze-film lubrication of a spherical synovial joint

A model of synovial fluid (SF) filtration by articular cartilage (AC) in a step-loaded spherical synovial joint at rest is presented. The effects of joint pathology (such as a depleted acetabular labrum, a depleted cartilage superficial zone consistent with early osteoarthritis and an inflammatory SF) on the squeezed synovial film are also investigated. Biphasic mixture models for AC (ideal fluid and elastic porous transversely isotropic two-layer matrix) and for SF (ideal and thixotropic fluids) are applied and the following results are obtained. If the acetabular labrum is able to seal the pressurised SF between the articular surfaces (as in the normal hip joint), the fluid in the synovial film and in the cartilage within the labral ring is homogeneously pressurised. The articular surfaces remain separated by a fluid film for minutes. If the labrum is destroyed or absent and the SF can escape across the contact edge, the fluid pressure is non-homogeneous and with a small jump at the articular surface at the very moment of load application. The ensuing synovial film filtration by porous cartilage is lower for the normal cartilage (with the intact superficial zone) than if this zone is already depleted or rubbed off as in the early stage of primary osteoarthritis. Compared with the inflammatory (Newtonian) SF, the normal (thixotropic) fluid applies favourably in the squeezed film near the contact centre only, yielding a thicker SF film there, but not affecting the minimum thickness in the fluid film profile at a fixed time. For all that, in the unsealed case for both the normal and pathological joint, the macromolecular concentration of the hyaluronic acid-protein complex in the synovial film quickly increases due to the filtration in the greater part of the contact. A stable synovial gel film, thick on the order of 10(-7)m, protecting the articular surfaces from the intimate contact, is formed within a couple of seconds. Boundary lubrication by the synovial gel is established if sliding motion follows until a fresh SF is entrained into the contact. This theoretical prediction is open for experimental verifications.

The material properties of the bovine acetabular labrum

The compressive and tensile material properties of the bovine acetabular labrum were measured. Confined compression testing was used to determine the aggregate compressive modulus and the permeability of the labrum. The compressive modulus of the labrum (0.157 +/- 0.057 MPa) is comparable to that of the morphologically similar meniscus, and approximately one-quarter to one-half that of the adjoining acetabular cartilage. The permeability of the labrum (4.98 +/- 3.43 x 10(-16) m4/N s) was lower than that of the meniscus and cartilage. With a significantly higher resistance to interstitial fluid flow across the acetabular rim than along the rim. Specimens from the posterior and superior regions of the labrum were tested to failure in uniaxial tension. The maximum stress at failure (11.9 +/- 6.1 MPa), maximum strain at failure (26.5 +/- 7.6%) and tangent modulus (74.7 +/- 44.3 MPa) were similar to those reported for the bovine meniscus, and to other tissues composed of highly oriented collagen fiber bundles. In tension, the labrum is much stiffer (10-15 x) than the adjoining articular cartilage, and the posterior region of the labrum is significantly stiffer (45%) than the superior region. The labrum's low permeability may contribute to sealing of the hip joint. The high circumferential tensile stiffness of the labrum, together with its ring structure, reinforce the acetabular rim and may contribute to joint stability.